Methods for joining structural steel can be split into three main categories; bolting, riveting, and welding.
Steel-to-steel connections in construction began to flourish in the 1920s, when steel-framed skyscrapers began to appear as symbols of urban progress and economic prowess. This continued through the 20th Century, with ever-larger, impressive and complex structures being built around the world.
There has been a shift in the methods used to join structural steel parts, as well as innovations in all three categories of joining technology.
Each method brings its own advantages and disadvantages, with riveting being the first technique to take hold in industry. However, in recent years, bolting and welding have become the primary methods for connecting structural steel.
While any of these techniques can be used on their own, it is also possible to combine techniques (most frequently bolting and welding) to overcome particular challenges.
In addition to these widely-used techniques there are some more recently emerging variants, such as the use of pins or clamps, which are suitable for certain applications.
Because of the safety-critical nature of most construction projects, it is important to ensure any joins are strong and durable. As a result, structural steel constructions need to comply with standards and requirements for their design and fabrication, such as BS EN 1090.
There are also specific standards related to different connections, such as BS EN 14399, which recommends the use of preloaded bolting assemblies on bridges as they are designed to withstand vibrations and dynamic loads. European regulations also require engineers to follow manufacturer guidelines when tightening bolt sets and determine which types of washers can be used.
Here we will explore each of the three categories of structural steel joining method, including advantages and disadvantages for each, before briefly looking at some variant methods:
1. Welding
Heat has been used to bond metals for centuries, but modern welding could be said to have begun in 1881 when Russian inventor Nikolay Bernados created the first gas-powered arc welder. Structural steel welding was first used for buildings in the 1920s and remains a key method for joining steel components today. Most forms of welding use heat to melt metals together, creating a strong, permanent bond. However, it requires skilled labour and specialised equipment.
Advantages of welding include:
- Able to be performed on-site
- Maintains strength of components
- Saves materials
- Can be automated in some instances for higher production efficiencies
- Forms strong, permanent bonds
- Relatively quiet process
Disadvantages of welding include:
- Can cause brittleness around the heat affected zone and can introduce other defects, such as slag inclusion or poor penetration
- Welding can result in residual stress and deformation, impacting material performance
- Subject to brittle fracture at low temperatures
- Low plasticity and toughness
- Requires skilled workers
- Requires welding equipment and consumables
- Parts cannot be disassembled without damage
2. Riveting
Riveting was a common method for joining structural steels during the early 20th century, using a combination of heat and pneumatic pressure to drive the rivets into place through pre-drilled holes in steel components. While earlier examples of riveted structures include the Eiffel Tower (built 1887-1889), their ability to create incredibly durable joints saw them grow in use on everything from bridges to sky-scrapers and aircraft. However, a number of limitations saw the use of rivets largely replaced by bolts for many industrial applications during the middle of the 20th century. These drawbacks included the need for a specialised rivet gun, the noise created by the process, the difficulty in removing rivets should a mistake be made, and the energy and time taken for riveting. Rivet removal can require the use of a specialised rivet removal tool, which adds additional cost to the process compared to bolts. In addition, welds can deliver the same structural stability as rivets, which also added to the decline of riveting for connecting steel-to-steel across industry from the 1950s on.
Advantages of riveting include:
- Creates reliable and durable connections suitable for heavy and load-bearing structures
- Can be used with non-steel materials
- No thermal impact from the process
- Can be removed with less damage caused than with welding
- Easy to inspect
Disadvantages of riveting include:
- High costs for labour and materials
- Heavier than equivalent welded parts
- Stress concentration around holes can lead to weakness
- Difficult / costly to remove rivets
3. Bolting
Bolts have become an important technology for joining structural steels because they are easy to use and replace, providing a temporary connection between parts without damaging them. It is also relatively simple to ensure a good join, compared to welding for example, and requires no specialist knowledge or equipment. However, despite the many advantages, there are some drawbacks associated with bolts, which can unscrew as a result of heavy vibrations in a structure such as a bridge. In addition, most bolting solutions require access to both sides of the join, although modern bolt innovations have made this less of a concern.
Advantages of bolting include:
- Easy and fast to install and disassemble
- Does not create noise (like rivets)
- Does not require skilled labour
- Parts are not damaged ahead of reuse or reassembly
Disadvantages of bolting include:
- Bolt holes need to be pre-drilled and aligned for assembly
- Bolt holes weaken the component’s cross-section and create areas of lower strength with high stress concentration
- Requires parts to overlap or auxiliary plates to be added, adding to the cost of the materials used
- Bolts can loosen as a result of vibration and shock, so will need to be inspected and tightened periodically
- Most bolts need to be accessible from both sides of the structure
Steel components can be pinned together to allow for some axial movement between the parts. They are used for trusses and other applications where some movement is required in a structure. Pins require pre-drilled holes through which the pins can be inserted before being secured on the other side.
Clamps can also be used in some circumstances to hold two pieces of structural steel, such as beams or girders, together without the need for welding, riveting, or causing damage to the parts. These specialised clamps can create a secure connection but will add weight to a structure.
It is common practice now to combine more than one of these methods in order to maximise the benefits and minimise any drawbacks. This is commonly achieved through a combination of welding and bolting.
The quality of welds can generally be controlled better in a workshop than on site, so it common for parts to be welded before transportation to site where they can be bolted together. This combination allows for structural steelwork parts to be assembled quickly and cost-effectively, requires less inspection requirements, allows for adjustments to be made on site as necessary, and means that parts are not so susceptible to poor weather conditions during assembly.
Combination connections are widely used in the construction industry.
Steel has been used for the construction of many of the world’s most famous structures including the Willis Tower in Chicago, New York’s Empire State Building, and the Lloyd's building in London.
Of course, structural steel can be found in a wide range of other structures, including bridges and other civil engineering projects, which require it to be joined in some way.
While there are some variants, such as clamping beams and pins, that can be used for specific applications, structural steel joining methods tend to be bolting, riveting, welding or a combination of them.
Each technique has its own advantages and disadvantages, meaning that using more than one method can maximise the advantages and reduce the drawbacks.
There are strict standards that govern the use of steel in critical projects such as buildings and it is important to choose the best method or methods for your given application. This will depend on service conditions and requirements, whether the structure is to be temporary or permanent, and more.